The present invention refers to the use of colloidal photocatalytic preparations of titanium, dioxide (TiO2) for maintaining the original appearance of cementitious, stone, or marble products.
The conservation of architectural assets depends to a considerable extent on the environment in which they are located. The problem of the preservation of such assets assumes important dimensions if it is considered that the action of atmospheric agents has become increasingly aggressive over time.
The conservation of architectural structures, buildings and artefacts subject to outdoor exposure has to do with forces that act in an important way on the surface is of the constructions or in the layers immediately below the surface and that regard the depositing of organic and inorganic materials which usually adhere to the aforesaid organic substrate, and the action of acid rain.
One of the main problems of architectural cementitious, stone, or marble products therefore regards the constant maintenance over time of their original appearance, which is undermined by the natural process of ageing due to the influence of atmospheric agents.
The need is therefore particularly felt for economical systems of intervention which enable conservation of the aesthetic characteristics of the surface of constructions made of cementitious, stone, or marble material.
With the aim of protecting such artefacts, various techniques are resorted to, the most common of which consist in applying water-repellent products, such as silicone solutions that are stable in alkaline environments and are stable to light and weather. Unfortunately, the progressive increase in the pollutants present in a typical metropolitan environment, perhaps also on account of their frequently acidic nature, has markedly limited the duration of this type of protection, with consequent aesthetic degradation of the artefact.
In order to overcome this new problem, various solutions have been proposed. For example, the patent IT 1286492 (in the name of the present applicant) illustrates a hydraulic binder for cementitious compositions which comprises in its mass a titanium-dioxide based photocatalyst that is able to oxidize, and hence neutralize, the polluting substances present in the environment. This type of solution is certainly valid as regards the protection of the surface of the artefact from any possible deterioration caused, for example, by wind abrasion, but clearly it implies the use of enormous amounts of photocatalyst as compared to those actually needed. In fact, the photocatalyst is present not only on the surface of the artefact or in the immediately underlying layers, i.e., in the areas where its presence proves useful, but in the entire cementitious mass, which, for the most part, will never come into contact with external agents.
Alternative solutions have also been proposed (see, for example, the European patent, application No. EP 0 885 857 in the name of the present applicant) which regard cementitious levelling plasters containing various polymeric additives and photocatalysts that are able to oxidize the polluting substances present in the environment, but these cementitious-based formulations are white, and cannot be used, for instance, on marble or stone materials without consequently modifying their aesthetic characteristics.
The use of anatase, which is one of the tetragonal crystalline forms of titanium dioxide, as photocatalyst for the oxidation of organic pollutants has been known for some time, also in the form of colloidal preparations.
The patent EP 784034 (in the name of Matsushita Electric Works) describes substrates obtained by depositing titanium dioxide on the surface of a substrate by deposition of a solution containing ammonium titanium fluoride, followed by calcining. The patent EP 614682 (in the name of Fuji Electric) illustrates a titanium-based or titanium/activated carbon-based photocatalyst fixed on a fluororesin to obtain sheets or panels to be applied externally on buildings for removing low concentrations of NOx.
The incorporation of metal ions (doping agents) in these preparations alters the photocatalytic activity of titanium dioxide to a substantial extent. The most important parameters are the type of ion, the concentration of the dopants, and the thermal treatment useful for the formation of the photocatalyst.
Brezovxc3xa1 V. et al., J. Photochem. Photobiol. A: Chem., 109, (1997), 177-183, analyze the influence of various metal ions and their concentrations in a particular application of titanium dioxide as photocatalyst, namely, in the oxidation of phenol. In particular, preparations of colloidal titanium dioxide are described, in which the titanium dioxide is doped with various metals at 5% atom/atom, following a procedure whereby colloidal titanium dioxide is first prepared, and next the salt of the doping metal is mixed under heating. Among the various metal ions tested, cerium is reported to reduce the photocatalytic activity of the colloidal titanium dioxide. In the publication, as in other previous publications on this subject, no mention is made of the use of the said photocatalytic products on cementitious materials.
The patent EP 633964 (in the name of Fujisawa, Hashimoto, and Ishihara Sangyo Kaisha) describes a TiO2 based photocatalyst preferably doped with V, Fe, Cu. Co, Ni, Zn, Ru, Rh, Pd. Ag, Pt, or Au, and fixed on a fluorinated polymer for adhesion to the substrate. This photocatalyst is useful for purifying air, but also water, from various undesired substances.
It has"" now been surprisingly found that colourless colloidal preparations of titanium dioxide, or of one of its precursors, possibly doped with elements chosen from groups I-VA, and the lanthanide and actinide series of the periodic table, or mixtures thereof, preferably magnesium, cerium, niobium or lanthanium, wh n applied on the surface of cementitious, stone, or marble products, are able to preserve the original appearance of the surface, without altering the characteristics of the cementitious, stone, or marble material.
The solution devised thus regards a system for treating cementitious, stone, or marble surfaces by using titanium dioxide-based colourless colloidal suspensions, the titanium dioxide being possibly doped with elements chosen from groups I-VA, and the lanthanide and actinide series of the periodic table.